# How would you explain an atomic mass number?

Sep 17, 2016

Well it's the sum of the massive nuclear particles, protons and neutrons.

#### Explanation:

As you know, a nucleus contains a given number of nuclides: (i) protons, massive positively charged particles; and (ii) neutrons, massive neutrally charged particles. The associated electronic mass is fairly insignificant in comparison.

The number of protons gives $Z$, the atomic number, which number defines the element: $H , Z = 1$; $H e , Z = 2$; $L i , Z = 3$; $B e , Z = 4$..................

However the nucleus may contain different numbers of neutrons, which difference gives rise to isotopes. For most hydrogen nuclei, where $Z = 1$ by definition, there is no neutron, and we represent this isotope as ""^1H, the protium isotope. A small number of hydrogen nuclei contain 1 neutron, and we represent this isotope as ""^2H, the deuterium isotope. A smaller number of hydrogen nuclei contain 2 neutrons, and we represent this isotope as ""^3H, the tritium isotope.

The weighted average of all isotopes is the mass number, and this number is quoted on the Periodic Table; of course you will never be expected to remember these numbers in an exam. You will be expected to utilize the Periodic Table provided to explain atomic numbers and mass numbers.